1. Field of the Invention
The present invention relates to integrated optical semiconductor light emitting devices, such as semiconductor laser diodes, arrays and light emitting diodes, and, in particular, to a new method of improving performance of the light emitting devices by modification of their material including increasing power, slope efficiency, thermal stability and decreasing threshold current and linewidth enhancement factor. The modification of the laser material improves performance of the semiconductor light emitting device by creating clusters (quantum dots) in its active or passive regions improving efficiency of the lasing material and changing its refractive index.
In one preferred embodiment, the change in refractive index as a result of the modification process is used to fabricate a longitudinally inhomogeneous waveguide structure along the active light emitting device in order to expand the beam preventing its self-focusing, to increase output power and to make the beam almost diffraction limited. In another preferred embodiment, the change in refractive index is used to isolate individual active elements of a diode laser array in order to suppress amplified spontaneous emission in the lateral dimension.
The method can be used in all applications where highly efficient diode lasers, diode laser arrays or light emitting diodes are required, including high-power laser diode systems for pumping solid state lasers, industrial cutting and welding and different medical procedures including photo-dynamic therapy.
2. Information Disclosure Statement
Semiconductor laser technology offers the most efficient and flexible means for generating coherent laser radiation at different wavelengths. It has the potential to replace, in the future, large inefficient and expensive laser systems in many industrial, scientific, medical and military applications. The benefits of semiconductor laser diodes include less expensive, more efficient and more compact laser delivery systems with substantial output power.
Many laser delivery systems use compact and inexpensive laser diodes or diode laser arrays as a source of coherent radiation. Combination of high brightness beams delivered by optical fibers from individual laser diodes would provide a flexible means for generating powerful output beams at various frequencies. Semiconductor technology has the potential to provide wavelengths in the visible to middle infrared regions at powers over 70 watts by combining beams from individual lasers. Therefore semiconductor lasers may successfully replace different lasers currently used in many industrial, medical and scientific applications.
Improving performance of the semiconductor lasers and reducing price per watt of delivered power is a critical problem to be solved for all applications. High brightness beam generated by diode lasers or diode laser arrays is required, for example, in different diode pumped solid state laser systems, laser radars, systems of free space laser communication, industrial cutting and welding and systems for various medical surgery, diagnostic and treatment, including photo-dynamic therapy.
In order to improve power, slope efficiency and thermal stability of the laser diodes and laser diode systems, and reduce their threshold current and linewidth enhancement factor, new active laser materials should be designed. At the beginning of the development of laser diode technology researchers used heterostructures (sandwiches created by alternating layers of two materials with different conductive properties) to reduce the movement of electrons and improve the laser performance. By further limiting the carriers, creating a multiple quantum wells (MQW) structure in the active region of the laser diode or laser diode array, researchers increased efficiency and thermal stability of diode lasers while reducing threshold currents. One of the most efficient method of further improving laser diode performance is creating microscopic quantum wires or three-dimensional clusters (quantum dots) in the active region of the semiconductor device. Fabrication of efficient quantum dot materials would revolutionize the laser diode technology leading to highly efficient threshold-less semiconductor laser emitters.
Today, the uniform island of high-quality semiconductors on the nanoscale can be created using highly strained semiconductors and standard, self-assembling deposition technique such as molecular beam epitaxy. By carefully choosing semiconductor material, one can create quantum dot structures capable of emitting visible light by using AlInAs/AlGaAs or infrared emissions using InGaAs/GaAs.
The present invention suggests new approach to improving performance of the semiconductor light emitting devices. It is based on a new method of modification of the device semiconductor material creating microscopic clusters (quantum dots) and changing refractive index of the material. The modification of semiconductor material is caused by a drift of intrinsic defects to the surface of semiconductor, in particular, by drift of the interstitial Ga atoms in GaAs crystal in the electric field induced by a near-surface band bending due to adsorption of appropriate dopants. On achieving the limit of solubility for the concentration of interstitial material in crystal, it forms clusters (quantum dots) which drastically change properties of the semiconductor material in its near-surface region. The creation of quantum dots in active semiconductor region increases power and slope efficiency of the semiconductor light emitting device, reduces its threshold current and linewidth enhancement factor, improves electrical efficiency of the system, simplifies heat sink requirements and increases the system lifetime and performance.
Moreover, extremely large index variation in the semiconductor material can be achieved as a result of the cluster formation. This provides a unique possibility for low cost fabrication of a broad class of semiconductor based active or passive diffractive optical elements. In one preferred embodiment, the cluster formation and index variation can be supported by illumination of the semiconductor material with the light beam at the fundamental absorption wavelength in presence of highly polar liquid having appropriate dopants dissolved in it. This provides a possibility of simple fabrication of the semiconductor based active or passive diffractive optical elements using simple process of direct laser beam writing.
U.S. patent application Ser. No. 08/749,814 describes a method of self-correction of the laser diode beam by creating a gradient index micro-lens near the output mirror of the laser diode employing the process of modification of refractive index under the influence of laser beam. The present invention, however, describes the more general process of modification of the semiconductor material and different applications of the process to improving the laser performance, particularly, by modifying the semiconductor material along the laser cavity.
In one preferred embodiment, the modification is employed to create quantum dots directly in active region of the semiconductor light emitting device, considerably improving its performance by increasing power, slope efficiency and decreasing threshold current.
In another preferred embodiment, the modification is used to control the refractive index distribution along active and passive regions of the semiconductor material in order to fabricate a longitudinally inhomogeneous waveguide structure along the laser device or isolate individual emitters in a laser diode array. Moreover, the process of modification of the refractive index in the vicinity of laser mirrors can be used instead of coating for controlling reflection from the mirrors. This also results in considerable increasing in the laser power and decreasing the laser threshold current.
The proposed method can be applied to all semiconductor based light emitting devices including standard horizontal cavity diode lasers, bars, arrays, VCSELs (vertical cavity surface emitting lasers) and also light emitting diodes.